Coater equipment based on applying the coat to the surface of a moving web by means of an unguided jet directed to the web surface are generally known in the art as jet nozzle applicators. In these jet nozzle applicators, the coating mix is applied to the web surface with the help of a separate jet-forming slit nozzle, whereby the equipment construction may be varied widely. The present invention concerns a jet-nozzle type applicator in which the coating mix is directed to the web surface as a narrow linear jet via a slit orifice extending over that portion of the cross-machine width of the web which is to be coated. This type of a coater is also known as a fountain coater. In the jet nozzle coater, or the fountain coater, the entire amount of required coat is transferred to the web surface. The application of the coat is performed while the surface of the web is running supported by a backing roll, and conventionally, the coat is smoothed immediately after application by means of a doctor blade adapted to the perimeter of the same backing roll. The operating parameters of the coating mix jet can be controlled by varying the slit opening width, jet angle and position of the slit orifice lips. This method of coat application is characterized by a very low loading of the web and relatively modest pumping volume of the coating mix, whereby the need for linear loading of the doctor blade remains smaller than in, e.g., a roll applicator, thus improving web runnability and increasing the life of the doctor blade.
In the European patent application No. 91306138.8 is described a jet nozzle applicator in which coat application occurs in the above-described manner. The apparatus comprises a rotating backing roll and a jet nozzle having a slit orifice adapted close to a web running supported by the backing roll. The coat is applied to the web through the slit orifice and subsequently doctored to the desired coat weight with the help of a doctor blade mounted close to the backing roll. The jet nozzle assembly is mounted to be rotatable such that the nozzle tilt angle with respect to the backing roll can be adjusted. The center of rotation for tilting of the nozzle assembly is arranged to be at the nozzle orifice exit opening, whereby the clearance of the nozzle orifice from the web remains constant during the adjustment of the nozzle angle with respect to the web.
Using a sufficiently high impact velocity and large volume of the coating mix jet stream, current jet nozzle coaters can be run at web speeds as high as about 1500-1600 m/min, however, with the penalty that the high mass flow rate of coat passed on the moving web to the doctor blade may readily cause the doctor blade to vibrate. To make doctoring easier, the amount of applied coating mix should be controlled to as close as possible to the final coat weight, which is difficult to accomplish in current coaters that do not lend themselves to operate at such low coat weights. However, if the amount of applied coat is kept below 150 g/m.sup.2, doctoring can be performed at a relatively low linear loading of the blade, which means that theoretically a jet nozzle applicator should be able to manage these coat weights also at high web speeds. Herein, another problem arises at high web speeds in that the coating mix jet fails at these low coat weights to penetrate sufficiently well through the air layer travelling with the web, whereby the web tends to become marked with a large number of uncoated spots. Since the velocity of the coating mix jet stream cannot be increased above a certain limit, it is necessary to use a large amount of applied coat to cut through the air barrier. If the jet velocity is reduced with regard to the web speed, the mass flow rate of coating mix passed through the jet nozzle may be increased correspondingly; but even in this case the coat amount fed through the jet nozzle cannot be made very small. Typically, the exit velocity of the coating mix jet stream is 15-20% of the web speed, which means that a web speed of 20 m/s requires a coating mix jet stream velocity of 4-5 m/s.
In a jet nozzle applicator, the amount of coat applied to the web is in the order of 200-300 ml/m.sup.2, from which the doctor blade removes 90-95%. Herein, the nozzle orifice gap width is set to about 0.7-1 mm, sometimes even as wide as 3 mm.
The smallest amount of coating mix that can be applied is determined by the gap width of the slit orifice. Obviously, while a narrower orifice allows a thinner coating mix jet stream to be ejected, a practical difficulty appears in the manufacture of straight lips for slit orifices having gap widths of less than one millimeter extending over the entire machine width with a high dimensional accuracy. In fact, to apply exactly the desired amount of coating mix to web, the gap width of the slit orifice should be controlled to 20-100 um. As the gap width of a slit orifice this narrow cannot be measured reliably by means of mechanical gauges, very expensive measurement devices of special design would be required for gauging the gap width of the slit orifice. On the basis of the above discussion, it is easy to see that, by virtue of a facility permitting direct application of a desired coat amount to the web, the doctoring step after the application of the coat would become redundant and also other benefits could be gained.